Modern electronic equipment such as televisions, telephones, radios and computers are generally constructed of solid state devices. Solid state devices are preferred in electronic equipment because they are extremely small and relatively inexpensive. Additionally, solid state devices are very reliable because they have no moving parts, but are based on the movement of charge carriers.
Solid state devices may be transistors, capacitors, resistors, and other semiconductor devices. Typically, such devices are formed in and on a substrate and are interconnected to form an integrated circuit. One type of transistor is the metal oxide semiconductor field effect transistor (MOSFET) in which current flows through a narrow conductive channel between a source and drain and is modulated by an electric field applied at the gate electrode. In tight pitch gate applications such as memory arrays, for example, MOSFETs may be packed within 2,000-3,000 angstroms of one another.
A MOSFET is typically fabricated by forming the gate electrode outwardly of a substrate. Dopants are implanted and diffused into the substrate on either side of the gate electrode to form the source and drain as well as pockets and extensions for the source and drain. Contacts connect each of the gate electrode, source, and drain to other components of the integrated circuit.
To minimize short channel effects that degrade device performance, the source and drain are spaced apart from the conductive channel underlying the gate electrode. This is generally accomplished during fabrication by enlarging the sidewall insulator formed along the gate electrode to act as a spacer and prevent source and drain dopant implant in the substrate next to the gate. For tight pitch applications having small device size, however, the enlarged sidewall spacer takes up a large fraction of the area between the adjacent gate electrodes. As a result, contact areas for the source and drains are reduced and device yield is decreased.